For a steam generator tubing (hereinafter, referred simply as a “SG tubing”) used for a nuclear power plant, chromium-containing austenitic alloys such as 600 alloys or 690 alloys have been used. This is because these alloys have excellent corrosion resistance in high-temperature water environments.
These members are used in an environment of high-temperature water of about 300° C., which is a nuclear reactor water environment, for several years to several tens of years. For the chromium-containing austenitic alloy used as a SG tubing for nuclear plant, although Ni is contained much, and therefore the corrosion resistance is excellent and the corrosion rate is low, a minute amount of Ni is released from a base metal by the long-term use.
In a process in which reactor water circulates, the released Ni is carried to a reactor core portion and receives neutron irradiation in the vicinity of fuel. When receiving neutron irradiation, Ni is converted into radioactive cobalt by nuclear reaction. This radioactive cobalt continues to release radiation for a long period of time because the half-life thereof is very long. Therefore, if the release amount of Ni is large, regular inspection cannot be started until the radiation dose emitted decreases to a proper value, so that the period of regular inspection extends, which results in an economic loss.
To reduce the amount of the released Ni from substrate is a very important issue in using a light water reactor for a long period of time. So far, therefore, by improving the corrosion resistance on the material side and by controlling the quality of reactor water, measures have been taken to suppress the release of Ni from the Cr-containing austenitic alloy.
Patent Document 1 discloses a method in which Ni-based alloy heat-transfer pipe is annealed in the temperature range of 400 to 750° C. in an atmosphere having a degree of vacuum of 10−2 to 10−4 Torr to form an oxide film consisting mainly of chromium oxides, whereby the general corrosion resistance is improved.
Patent Document 2 discloses a method for producing a member for nuclear plant, in which after the solution treatment of a Ni-based precipitation strengthened alloy, heating treatment is performed as at least part of age hardening treatment and oxide film forming treatment in an oxidizing atmosphere of 10−3 Torr to atmospheric pressure.
Patent Document 3 discloses a method for producing a Ni-based alloy product, in which a Ni-based alloy product is heat-treated in an atmosphere of hydrogen or a mixed atmosphere of hydrogen and argon, the atmosphere having a dew point of −60° C. to +20° C.
Patent Document 4 discloses a method for Miming a chromium-rich layer by exposing an alloy workpiece containing nickel and chromium to a gas mixture of water vapor and at least one kind of non-oxidizing gases.
Patent Document 5 discloses a method for producing a Cr-containing nickel-based alloy pipe, in which the Cr-containing nickel-based alloy pipe is treated in an atmosphere consisting of non-oxidizing gas containing oxidizing gas, whereby a chromium oxide film having a predetermined thickness is formed on the inner surface of pipe.